A portable telephone system, such as the second-generation digital cordless telephone (CT2), has multiple transceivers which are located at a call point station or public base station known as a telepoint. These transceivers allow persons using portable telephones or cordless handsets to access the public telephone network when in range or within a service area after the cordless handset has established an asynchronous link with the base system.
In the CT2 system, the cordless handset initiating a call, asynchronously transmits on one available channel of the handset's transceiver which corresponds to a transceiver radio frequency (RF) channel of the base.
According to one protocol out of other applicable protocols, called multiplex 3 (MUX3) of the CT2 specification, the channel used by the cordless handset is first divided in the time domain into 7 frames, F1-F7, as illustrated in FIG. 1a. Accordingly, the cordless handset transmits continuously for five frames or transmission bursts of ten milliseconds and the receiver receives for four milliseconds in a receiving time-slot window, when the transmitter is turned off for two frames. Within the ten milliseconds of transmit time comprising 5 frames, the information is repeated four times (in each submux) within a two millisecond frame, as seen in FIG. 2.
A typical layout of the MUX3 format in a sequential order of the frames is shown in FIG. 2. The labels F1 through F7, respectively, indicate frame numbers. The period of a single frame is two milliseconds. Referring to both FIG. 1b and 2, each frame, F1-F7, is divided into four smaller subframes s1-s4 containing differently sized data (D) or frame synchronization words (CHMP), each preceded by a different number of preamble bits (P). The entire synchronization and data information of the repeating data signal 16 is then repeated four times in each of the four subframes s1-s4.
All the subframes, where the complete information is periodically provided once, are grouped together to form a submultiplex or submux 40. Each frame thus consists of four submuxes 40, corresponding to the subframes s1-s4. Within each submux 40, a D channel synchronization character (SYNCD), 3 address code words (ACW), and a frame synchronization character of twenty-four bits (CHMP) exist. Each of these information words are preceded by some number of preamble bits. The D channel synchronization character (SYNCD) precedes and are grouped with the address code word (ACW) to form the D channel data. All of the D channel data are subdivided into separate data words of ten bits each. Thus, every twenty bits of the D channel data are repeated in each of the four submuxes 40 before the D channel data changes.
Each subframe or submux consists of thirty-six bits, in a row. Thus within the subframe or submux s1 of frame 1 (F1), the first bit is reserved for the first bit a 6 bit preamble word of the first SYNCD 10 bit data word. An 8 bit preamble word precedes the second 10 bits SYNCD data word. Finally, the first two bits of the 8 bit preamble word of the next 10 bit SYNCD data word ends the first row of the first submux or subframe s1. The rest of the D channel data are arranged accordingly, as seen in FIG. 2.
On the other hand, within any of the submuxes or subframes of frame 5 (F5), the 24 bit frame synchronization character (CHMP) is preceded by a 12 bit preamble word. Because a smaller number of preamble bits (P) precedes the D channel data (D) than the frame synchronization character (CHMP), the frame synchronization character (CHMP) is misaligned with and lags the D channel data (D).
As is known, the preamble bits are used to first synchronize an internal clock of each receiver of the base transceiver to that of the received signal from the cordless handset so that subsequent proper decoding of the synchronization code may be achieved. Even though the frame synchronization character (CHMP) occurs at the end of the transmission burst, the frame synchronization character usually needs to be detected first since it denotes the start of important information and has a low probability of falsing.
On the other end as seen in FIG. 1b, the base station asynchronously receives (14) and transmits (12) alternately for one millisecond in a time domain duplex (TDD) burst mode, after a synchronous link has been established with another CT-2 protocol MUX1 or MUX2. While scanning through the different radio frequencies of its transceivers, the base station looks for the presence of a synchronization signal (i.e. the frame synchronization character CHMP) to determine if that channel or frequency is used by a handset in a call attempt.
Hence, after synchronizing the internal clock of the base to the cordless handset by synchronizing to the preamble bits of the frame synchronization character CHMP and validating future data by correlating the correct frame synchronization character CHMP, the base may activate its receiver to periodically start the receive window to align to the remaining D channel information or data (D) according to at the point where the incoming frame synchronization character was received. However, portions of the data words would be lost at the beginning of this receive window due to the misalignment of the start of the frame synchronization character (CHMP) and the D channel data (D).
Furthermore, slight phase differences between the transmit frame of the cordless handset and the receive window of the base station can cause further misalignment. This misalignment eventually results in the receiver of the base station receiving the frame synchronization character too early in the received window, as discussed previously. On the other hand, the frame synchronization character can be received too late as to where the last bits of the incoming frame containing the synchronization character CHMP is received at the antenna of the base station but lost at the beginning of a new transmit burst due to the internal propagation delay of the receiver.
Since the base receive window of one millisecond can fit up to two submuxes as seen in FIG. 1b, the duplicative information in the submuxes can corrupt, needlessly, the operation of AGC receiver, such as a zero intermediate frequency (ZIF) receiver disclosed in a co-pending U.S. patent application Ser. No. 7/574,628 and incorporated herein by reference. For example, if the signal is strong, the falsing rate in an AGC receiver may be high due to a need for AGC re-acquisition or stabilization, after the AGC has been discharged during the time when valid data is not present. Accordingly, there exists a need to provide a reliable method of receiving asynchronous data in a multi-transceiver synchronous burst mode system while optimizing the operation of the receiving circuits of the base transceiver which includes both the receiver and detector.